1. Field of the Invention
The present invention relates to a novel redox catalyst system, and a process for preparing dispersions by emulsion polymerization of vinyl compounds with the aid of this catalyst system. Furthermore, the present invention is directed to dispersions, and in particular, to those based on polyvinyl esters, which have been prepared with the aid of a catalyst system of the present invention and also to the use of the dispersions as water-resistant and formaldehyde-free adhesives for porous and semiporous substrates.
2. Description of the Related Art
Redox catalyst systems are widely used for the initiation of the free-radical emulsion polymerization of vinyl compounds. Their advantages compared with single-component thermally dissociative initiators include a short induction period and a low activation energy which allows polymerization at low temperature and results in high molecular weights of the polymers. Numerous combinations of redoxcatalyst systems have been described both in the scientific literature and in the patent literature. Frequently used redox pair combinations comprise the oxidizing agents hydrogen peroxide, benzoyl peroxide, alkyl hydroperoxides, peroxodisulfates on one hand. On the other hand, reducing agents such as transition metal ions may also be used. Example of reducing agents include Fe.sup.2+, carboxylic acids such as ascorbic acid or tartaric acid, sulfur compounds such as sodium hydrogen sulfite, zinc or sodium formaldehyde sulfoxylate, and the like. Comprehensive reviews of redox polymerizations may be found in the technical literature, for example by G. S. Misra and U. D. N. Bajpai in Prog. Polym. Sci. 8, 61-131 (1982).
For preparing homopolymeric and copolymeric polyvinyl- ester dispersions, for example for the adhesives sector, use is likewise frequently made of redox catalyst systems for the initiation of the polymerization. In particular, systems which are stabilized with protective colloids such as polyvinyl alcohol may be used. This is made clear by the documents DE-A 44 20 484, which discloses tert-butyl hydroperoxide/sodium formaldehyde sulfoxylate, EP-A 0 623 661, which discloses ammonium peroxodisulfate/Fe.sup.2+ /sodium formaldehyde sulfoxylate, and DE-A 39 42 628, EP-A 0 394 774 and DE-C 26 20 738 which all disclose tert-butyl hydroperoxide/sodium formaldehyde sulfoxylate. According to DE-C 26 20 738, the use of a redoxcatalyst system, viz., tert-butyl hydroperoxide/sodium formaldehyde sulfoxylate, offers the particular advantage of an improvement in the water resistance of the film derived from the dispersions. This improvement is achieved by means of a grafting reaction of the vinyl ester onto the protective colloid polyvinyl alcohol.
The use of redox catalyst systems for preparing vinyl acetate-ethylene emulsion copolymers in which the reducing component can be a bisulfite adduct of a carbonyl compound is generally known. Examples include a ketone, glyoxal or a glyoxal derivative such as methylglyoxal or else an adduct of various further sulfur compounds with glyoxal or methylglyoxal. DE-A 32 39 212 and U.S. Pat. No. 4,360,632 describe the preparation of lattices with the aid of a formaldehyde-free redoxcatalyst system. This catalyst system comprises an oxidizing agent such as tert-butyl hydroperoxide, and a water-soluble adduct of sodium hydrogen sulfite with a ketone having from 3 to 8 carbon atoms as reducing agent, such as acetone-sodium hydrogen sulfite. The monomer system preferably used comprises vinyl acetate, ethylene and further functional comonomers.
The documents DE-A 26 49 532, JP-A 55 709/77 and JP-A 99 096/85 relate to formaldehyde-free redoxcatalyst systems which contain, apart from an oxidizing agent, reaction products of glyoxal or methylglyoxal with reducing sulfur/oxygen compounds. Examples of reducing sulfur/oxygen compounds disclosed include alkali metal, ammonium or zinc salts of thiosulfuric acid, dithionous acid, disulfurous acid, sodium hydrogen sulfite or potassium hydrogen sulfite. Specific compounds used are alkali metal hydrogen sulfite adducts of glyoxal. The polymers concerned are vinyl acetate-ethylene copolymers which may contain further crosslinkable copolymers such as n-butoxymethylacrylamide. The products are suitable as formaldehyde-free binders for nonwovens and paper, but do not possess the desired degree with respect to water resistance can be achieved using glyoxal derivatives (See Comparative Example 1).
Furthermore, U.S. Pat. No. 3,438,941 discloses the use as a reducing agent of an adduct of sodium hydrogen sulfite and polyacrolein acidified to a pH of less than 1.5 using hydrochloric acid. These compounds are used in combination with an oxidizing mixture of ammonium persulfate and tert-butyl hydroperoxide, specifically for the emulsion polymerization of .alpha.,.beta.-unsaturated aldehydes such as acrolein. The adduct used as reducing agent simultaneously functions as stabilizer when correspondingly larger amounts are used. However, due to the low pH level, the polymerization conditions described in U.S. Pat. No. 3,438,941 are unsuitable for the polymerization of vinyl esters.
In addition, the relatively recent literature describes work on the development of redoxcatalyst systems for emulsion polymerization of vinyl esters in the presence of polyvinyl alcohol functioning as a protective colloid, particularly pure vinyl acetate. Bisulfite adducts of monofunctional carbonyl compounds are used as reducing components in combination with peroxodisulfates. In Pigment and Resin Technology, issue July/August, pp. 4-6, 17 (1993), R. M. Mohsen et al. describe the preparation of polyvinyl acetate dispersions stabilized with polyvinyl alcohol as adhesives with the aid of the redoxcatalyst system potassium peroxodisulfate/octanal (sodium hydrogen sulfite). A. S. Badran et al. describe emulsion polymerizations of vinyl acetate in the presence of polyvinyl alcohol with the aid of the various catalyst systems. Potassium peroxodisulfate/salicylaldehyde (sodium hydrogen sulfite) is described in Acta Polym. 42, 1 (1991). Potassium peroxodisulfate/ acetone(sodium hydrogen sulfite) is described in Acta Polym. 41, 187 (1990). Potassium peroxodisulfate/cyclohexanone (sodium hydrogen sulfite) is described in J. Polym. Sci. A 28, 411 (1990). Redoxcatalyst systems comprising the reducing agents benzaldehyde (sodium hydrogen sulfite), acetaldehyde-(sodium hydrogen sulfite) and (methyl propyl ketone)-(sodium hydrogen sulfite) are described by A. S. Badran et. al. in J. Appl. Polym. Sci 49, 187 (1993). A disadvantage of the use of monofunctional bisulf ite adducts as reducing agents in redox systems is that, as described above with respect to many other possible systems, such adducts are not capable of attaining the desired degree with respect to the water resistance in the gluing of wood (See Comparative Example 3).
Polyvinyl ester dispersions which are used as adhesives for semiporous and porous substrates are employed for water-resistant bonding in accordance with standards. Subsequent addition of various crosslinkers, can greatly improve the water resistance of the films derived from the polyvinyl ester dispersions, even to boiling water. In particular water resistance is increased in the presence of Lewis acids, compounds which can be complexed by the protective colloid, for example aluminum(III) salts, with the aqueous phase becoming acid. For example, masked polyfunctional aldehydes having at least 3 carbon atoms as taught by DE-A 44 20 484, which is non-prior art, can be used. In addition, the use of masked polyfunctional isocyanates as taught by EP-A 0 206 059, crosslinkable comonomers such as N-methylolacrylamide or related compounds as taught by DE-C 26 20 738, DE-A 39 42 628 and EP-A 0 394 774 or a combination of both measures as disclosed in DE-A 44 20 484 and EP-A 0 623 661 are possible.
In recent times, for toxicological reasons, efforts are being made in industry to avoid where possible the use of components which contain formaldehyde in free or bound form as depot formaldehyde which can be liberated in a crosslinking step. Examples of such components include formaldehyde resins, N-methylol(meth)acrylamide as monomer or sodium formaldehyde sulfoxylate as constituent of a redox catalyst system. This also applies to the adhesives sector and the glue sector.